As for me i have 5 germans on my ship and 2 brits " 2nd engeneer "on the support
slots and there is not mutch difference between them when it comes to gain exp.

1) you can mix them, and they get FULL experience for what they do i.e. gunners get full
gun exp, and torpers get full torp exp (trust me, i know) remember folks, generally when
your leveling sailors on another nation your crew isnt the same level, which applies
penalties and the like

2) they provide NOTHING, the only advantage of having them there is getting experience
(and some experts) on them without having to use their nation (again tested)

3) thats why so many people torp whore crews of other nations on fubuki (i do it too)...
because there is NO real disadvantage over leveling sailors of the same nation

I'm training a german and an american gunner on my brit ship.

i am training a jap gunner for someone on my US ship.

when I my guys make 1300 expereince, the jap gets 1000. So its not that big of a difference.

Sailors on a ship (or support slot) of another nation will not gain full EXP nor bring
their full ability to
bear (read: they aren't working as good as on a ship of their own nation)